Single-site polyethylene has narrow molecular weight distribution and uniform composition distribution (i.e., the comonomer recurring units are uniformly distributed along the polymer chains). The combination of narrow molecular weight distribution and uniform composition distribution distinguishes single-site polyethylene from conventional polyethylene made by Ziegler or chromium catalysts. Compared to Ziegler polyethylene, single-site polyethylene has improved impact resistance, tensile strength, and optical properties.
However, the uniformity of molecular weight distribution causes reduced thermal processability of single-site polyethylene. It is difficult to process single-site polyethylene under the conditions normally used for Ziegler polyethylene. The reduced processability limits the development of single-site polyethylene because the alteration of the process conditions requires a large capital investment. Accordingly, it would be highly desirable to prepare polyethylene which possesses the improved physical properties offered by single site catalysts and also exhibits processability characteristics which are similar to those of conventional polyethylene.
One approach to achieve this object is by using mixed catalyst systems. For instance, U.S. Pat. No. 5,747,594 teaches a two-stage polymerization process. In a first stage, ethylene and a higher α-olefin are polymerized with a single-site catalyst. The polymerization continues in a second stage where a Ziegler catalyst is used. Therefore, the product is a mixture of single-site polyethylene and Ziegler polyethylene. The disparity of the two polymers in molecular weight and composition gives the product an improved thermal processability.
Another alternative is using a single-site catalyst in two polymerization reactors which are operated with different activators. For instance, an alumoxane is used in one reactor and an ionic activator is used in the other. The use of different activators results in polyethylene made in the different reactors having different molecular weights and thus the combined polyethylene has a broad molecular weight distribution and an improved processability. See U.S. Pat. No. 6,372,864.
However, the use of mixed catalysts or activators is generally associated with operability problems. The two different catalysts or activators may interfere with one another, for example, the organoaluminum compounds which are often used in Ziegler catalyst poison single-site catalysts. Therefore, catalyst deactivation is often involved when two incompatible catalyst systems are used. Catalyst deactivation is costly and complicated. See U.S. Pat. Nos. 5,371,053 and 5,442,019. Further, while mixing single-site polyethylene with Ziegler polyethylene may improve the processability, it also reduces the property characteristics of the single-site polyethylene.
Multimodal polyethylene can be made by a dual process using only single-site catalyst. For instance, co-pending application Ser. No. 10/462,493 teaches a dual olefin polymerization process. The process uses a bridged indenoindolyl ligand-containing Group 4 transition metal complex and an activator. It is carried out in multiple stages or in multiple reactors. The same complex and the same activator are used in all stages or reactors. Different polyethylenes are made in different stages or reactors by varying the monomer compositions, hydrogen concentrations, or both. The dual process usually can conveniently produce a bimodal resin. Given that each mode has relatively narrow molecular weight distribution, the bimodal resin, nevertheless, lacks optimal processability.
In sum, new process for producing single-site polyethylene is needed. Ideally, the process would use two or more single-site catalysts and produce polyethylene that has more than two modes.